XQuery An XML Query Language

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XQueryAnXML Query Language

• Samuel R. Collins

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Outline

• Introduction
• XML Review
• XQuery Overview
• Derivation of XQuery
• XQuery Expressions
• Examples
• Questions and Answers

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What is XML?

• The Extensible Markup Language (XML)
• A subset of SGML
• XML is a family of technologies
• Namespaces
• XML Schema
• XML Query
• XLink
• XPointer
• XSL
• XPATH

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How is XML Used?

• XML is an extremely versatile markup language,
  capable of labeling the information content of
  diverse data sources including
  
• Structured and semi-structured documents
• Relational databases
• Legacy databases
• Object repositories
• Database views
• Electronic data interchange (EDI)
• Facilitate B2B communication
• Data exchange between data sources

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Why Not SQL or OQL?

• XML Data is different from relational and
  object-oriented data
  
• In the relational and object-oriented models,
  every data instance has a schema, which is
  separate from and independent of the data.

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Why Not SQL or OQL?

• In XML, the schema exists with the data
• XML data is self describing
• XML data can be parsed without a DTD or an XML
  Schema

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XML Query Language

• A tool for structural and content-based query
• Allows an application to extract information from
  one or several data sources

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XQuery

• XQuery is designed to be broadly applicable
  across all types of XML data sources.
  
• It is designed to be a small, easily
  implementable language in which queries are
  concise and easily understood.
  
• It is also flexible enough to query a broad
  spectrum of XML information sources, including
  both databases and documents.
  
• The Query Working Group has identified a
  requirement for a human-readable query syntax and
  XQuery is designed to meet this requirement.

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XQuery Derivation

• XQuery is derived from an XML query language
  called Quilt, which in turn borrowed features
  from several other languages.
  
• From XPath and XQL it took a path expression
  syntax suitable for hierarchical documents.
  
• From XML-QL it took the notion of binding
  variables and then using the bound variables to
  create new structures.
  
• From SQL it took the idea of a series of clauses
  based on keywords that provide a pattern for
  restructuring data (the SELECT-FROM-WHERE pattern
  in SQL).
• From OQL it took the notion of a functional
  language composed of several different kinds of
  expressions that can be nested with full
  generality.
• Quilt was also influenced by other XML query
  languages such as Lorel and YATL.

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Additional Features of XQuery

• XQuery has a type system that is based on XML
  Schema.
  
• XQuery relies on path expressions for navigating
  in hierarchic documents.

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XQuery Expressions

• The principal forms of XQuery expressions are as
  follows
  
• Path expressions
• Element constructors
• FLWR expressions
• Expressions involving operators and functions
• Conditional expressions
• Quantified expressions
• Expressions that test or modify datatypes

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Path Expressions

• One of the forms of an XQuery expression is a
  path expression, based on the syntax of XPath.
  
• XPath is a notation for navigating along "paths"
  in an XML document, and is used in several
  XML-related applications including XSLT and
  XPointer.

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Path Expression Example

• The following example uses a path expression
  consisting of three steps.
  
• The first step locates the root node of a
  document.
  
• The second step locates the second chapter of the
  document
  
• The third step finds figure elements occurring
  anywhere within the chapter, but retains only
  those figure elements that have a caption with
  the value "Tree Frogs."

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Path Expression Example

• (Q1) In the second chapter of the document named
  "zoo.xml", find the figure(s) with caption "Tree
  Frogs".
  
• document("zoo.xml")//chapter2//figurecap
  tion "Tree Frogs"

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Path Expression Example

• XPath allows a node to be selected from a
  sequence of nodes by specifying its ordinal
  number in the sequence
  
• (Q2) Find all the figures in chapters 2 through 5
  of the document named "zoo.xml."
  
• document("zoo.xml")//chapter2 TO 5//figure

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Element Constructors

• A query often needs to generate new elements. The
  simplest way to generate a new element is to
  embed the element directly in a query using XML
  notation.
• (Q7) Generate an element that has an
  "empid" attribute and nested and
  elements.
  
• John Smith
• Anthropologist

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Element Constructors

• (Q8) Generate an element that has an
  "empid" attribute. The value of the attribute and
  the content of the element are specified by
  variables that are bound in other parts of the
  query.
• name
• job

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FLWR Expressions

• A FLWR (pronounced "flower") expression is
  constructed from FOR, LET, WHERE, and RETURN
  clauses, which must appear in a specific order.
  
• A FLWR expression binds values to one or more
  variables and then uses these variables to
  construct a result.
  
• The overall flow of data in a FLWR expression is
  illustrated in Figure 1.

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FLWR Expressions

• The first part of a FLWR expression consists of
  FOR-clauses and/or LET-clauses, which serve to
  bind values to one or more variables.
  
• A FOR-clause is used whenever iteration is
  needed.
  
• The result of the FOR-clause is a sequence of
  tuples, each of which contains a binding for each
  of the variables in the FOR-clause.
  
• A LET-clause is also used to bind one or more
  variables to one or more expressions. Unlike a
  FOR-clause, however, a LET-clause simply binds
  each variable to the value of its respective
  expression
• A FLWR expression may contain several FOR and
  LET-clauses. Expressions used in FOR and
  LET-clauses may contain references to variables
  bound earlier in the FLWR expression. The result
  of the FOR and LET clauses is an ordered sequence
  of tuples of bound variables.

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FLWR Expressions

• The binding-tuples generated by the FOR and LET
  clauses are subject to further filtering by an
  optional WHERE-clause.
  
• Only those tuples for which the condition in the
  WHERE-clause is true are used to invoke the
  RETURN clause.
  
• The WHERE-clause may contain several predicates,
  connected by AND and OR. These predicates usually
  contain references to the bound variables.
  
• The ordering of the binding-tuples generated by
  the FOR and LET clauses is preserved by the
  WHERE-clause.
  

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FLWR Expressions

• The RETURN-clause generates the output of the
  FLWR expression, which may be any sequence of
  nodes or primitive values.
  
• The RETURN-clause is executed once for each tuple
  of bindings that is generated by the FOR and
  LET-clauses and satisfies the condition in the
  WHERE-clause, preserving the order of these
  tuples.
• The results generated by the individual
  executions of the RETURN clause are concatenated
  together, preserving their order.

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FLWR Expressions

• (Q11) List the titles of books published by
  Morgan Kaufmann in 1998.
  
• FOR b IN document("bib.xml")//book
• WHERE b/publisher "Morgan Kaufmann
• AND b/year "1998"
• RETURN b/title

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FLWR Expressions

• (Q12) List each publisher and the average price
  of its books.
  
• FOR p IN distinct(document("bib.xml")//publisher
  )
  
• LET a avg(document("bib.xml")//bookpublisher
  p/price)
  
• RETURN
• p/text()
• a

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Sorting

• A sequence can be ordered by means of a SORTBY
  clause that contains one or more "ordering
  expressions."
  
• (Q17)List all books with price greater than 100,
  in order by first author within each group of
  books with the same first author, list the books
  in order by title.
• document("bib.xml")//bookprice 100 SORTBY
  (author1, title)

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Operators and Expressions

• Arithmetic operators
• AdditiveExpr      Expr ("" "-")
• ExprMultiplicativeExpr      Expr ("" "div"
  "mod") Expr
  
• UnaryExpr      ("-" "") Expr
• Comparison operators
• EqualityExpr      Expr ("" "!" ""
  "!") Expr
  
• RelationalExpr      Expr (""
  "") Expr
  
• Logical operators
• OrExpr      Expr "or" Expr
• AndExpr      Expr "and" Expr

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Operators and Expressions

• Sequence-related Operators
• ParenthesizedExpr      "(" ExprSequence? ")"
  
  
• ExprSequence      Expr ("," Expr)
• RangeExpr      Expr "to" Expr
• UnionExpr      Expr ("union" "") Expr
• IntersectExceptExpr      Expr ("intersect"
  "except") Expr
  
• BeforeAfterExpr      Expr ("before"
  "after") Expr

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Operators and Expressions

• (Q19) Prepare a "critical sequence" report
  consisting of all elements that occur between the
  first and second incision in the first procedure.
  
• LET p //procedure1
• FOR e IN // AFTER (p//incision)1
• BEFORE (p//incision)2
• RETURN shallow(e)
• The shallow function makes a shallow copy of a
  node, including attributes but not including
  subelements.
  

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Conditional Expressions

• Syntax
• IfExpr      "if" "(" Expr ")" "then" Expr
  "else" Expr
  
• (Q21) Make a list of holdings, ordered by title.
  For journals, include the editor, and for all
  other holdings, include the author.
  
• FOR h IN //holding
• RETURN
• h/title,
• IF (h/_at_type "Journal")
• THEN h/editor
• ELSE h/author
• SORTBY (title)

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Quantified Expressions

• The "some" expression uses an existential
  quantifier, and the "every" expression uses a
  universal quantifier.
  
• SomeExpr      "some" Variable "in" Expr
  "satisfies" Expr
  
• EveryExpr      "every" Variable "in" Expr
  "satisfies" Expr
  
• (Q22) Find titles of books in which both sailing
  and windsurfing are mentioned in the same
  paragraph.
  
• FOR b IN //book
• WHERE SOME p IN b//para SATISFIES (contains(p,
  "sailing") AND contains(p, "windsurfing"))
  
• RETURN b/title

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Datatytpes

• XQuery has a type system that is based on XML
  Schema. By using the datatype names defined in
  the namespace http//www.w3.org/2001/XMLSchema
  (hereafter abbreviated as xsd), all the primitive
  and derived datatypes of XML Schema can be used
  in queries.
• Other complex types declared using XML Schema can
  also be referred to by their qualified names.
  
• Literal values of XML Schema types other than
  string, integer, decimal, and float can be
  specified by means of constructor functions such
  as
• true()
• false()
• date("2000-06-25")
• or by cast expressions such as CAST AS
  xsdpositiveInteger(47).

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Any Questions
http//www.w3.org/TR/2001/WD-xquery-20010607